1. Field of the Invention
The present invention relates to a method of manufacturing a CMOS image sensor, and more particularly to a method of manufacturing a CMOS image sensor making it possible to form micro-lenses having a uniform shape throughout a semiconductor substrate.
2. Description of the Related Art
As generally known in the art, an “image sensor” is a semiconductor device, such as a CCD (Charge Coupled Device), a CMOS image sensor, etc., which converts optical image signals into electrical signals. The CCD has respective MOS capacitors positioned very close to each other for storing and delivering electrical charge carriers. The CMOS image sensor has adapted a switching mode by making MOS transistors as many as pixels with a CMOS technology, which uses control circuits and signal-processing circuits as peripheral circuits, and by detecting outputs in turn with the MOS transistors. In making such an image sensor, in the mean time, efforts are being made to improve the photosensitivity of the image sensor.
In general, the CMOS image sensor is composed of a photo-sensing means for sensing light and a CMOS logic circuit for processing sensed light into electrical signals to make them as data. For better photosensitivity, a Fill Factor, which is a ratio between the area of the photo-sensing means and the whole area of the image sensor, should be increased.
In the case of the CMOS image sensor, however, any attempt to increase the area of the photo-sensing means, under the restricted whole area of the sensor, has its limit because of a basic reason that the CMOS logic circuit cannot be eliminated.
As an alternative to overcome such problems, a photo-gathering technology has been widely studied that changes the path of light incident on an area other than that over the photo-sensing means and gathers the light into the photo-sensing means for better photosensitivity. An example of the photo-gathering technology is to make convex micro-lenses on top of the photo-sensing means using a material with good photo-transmittance and refract the path of incident light so that more light is transmitted to the photo-sensing means.
The structure of a CMOS image sensor using micro-lenses according to the related art will now be explained with reference to FIG. 1. The CMOS image sensor includes: a photo-sensing sensor 1 composed of photo-gates or photo-diodes, etc., to sense light and output electrical signals; a photo-shielding layer 2 made of a metallic layer to prevent any incidence of light on an area other than the photo-sensing area of the photo-sensing sensor 1; an inter-layer insulating film 3 formed on the photo-shielding layer 2; a color filter layer 4 made of dyed photoresist to transmit and deliver light with a specific wavelength to the photo-sensing sensor 1; a planarization layer 5 formed on the color filter layer 4 to overcome faults, steps or surface irregularities in the color filter layer 4; and micro-lenses 6 of polymer-based resin formed on the planarization layer 5 to gather light.
In the case of the CMOS image sensor using micro-lenses as configured above, any light parallel to the optical axes of the micro-lenses is refracted by them and focused at a location on the optical axes. Since one image sensor has tens of thousands of micro-lenses, a clear image is obtained only when the characteristics of respective micro-lenses have the same effect. Accordingly, the performance of the micro-lenses play a crucial role in the quality of image sensors.
Processes for manufacturing micro-lenses in the case of the CMOS image sensor of FIG. 1 will now be described with reference to FIGS. 2A to 2F.
First, as shown in FIG. 2A, a color filter layer 12 is formed on a substrate 11 provided with photo-sensing elements (not shown), such as photo-diodes, of an image sensor. Then, a photoresist 13 for forming the planarization layer is formed on the color filter layer 12. The photoresist for forming the planarization layer has the characteristics of low-sensitivity and high-transmittance in relation to a light source used in an exposure process during photolithography. Specifically, a negative-type photoresist is used.
Next, a reticle 15 is used to expose a predetermined area of the photoresist 13 to light for forming the planarization layer, as shown in FIG. 2B. The area exposed to light corresponds to the area where micro-lenses arc to be formed in subsequent process steps (a cell area) and the area not exposed to light corresponds to the area where pads arc to be formed in subsequent process steps (a peri, or peripheral, area). The photoresist in the area not exposed to light is then developed and removed to complete the planarization layer 14.
Thereafter, a photoresist layer 16 for forming micro-lenses is applied on the whole surface of the substrate, including the planarization layer 14, as shown in FIGS. 2D and 2E. A reticle 18 is then used to expose the photoresist 16 for forming micro-lenses to light selectively. A positive-type photoresist is used as the photoresist for forming micro-lenses. The area not exposed to light corresponds to the area where micro-lenses are to be formed.
In this state, the areas of the photoresist 16 exposed to light are developed and removed, as shown in FIG. 2F. Next, although not shown in the drawings, a baking process may be performed at a temperature of about 150° C. In this process, cuboid-shaped bodies of photoresist 17 melt to form convex micro-lenses that maintain their convex shape upon subsequent cooling.
According to the related method of manufacturing a CMOS image sensor, the planarization layer to be formed on the color filter layer cannot be formed on the peri area but on the cell area only. This causes a topographical fault, or step, between the peri area and the cell area. As a result, the photoresist for forming micro-lenses on the planarization layer generally does not have a uniform thickness over the entire surface of the substrate. A fault (believed to be caused by poor step coverage of the photoresist) then occurs at the interface between the peri area and the cell area.
Consequently, the final micro-lenses do not necessarily all have the same shape. In other words, uniform characteristics of all the micro-lenses cannot be guaranteed. The area where such irregularly shaped micro-lenses are formed is the interface between the cell area and the peri area, as mentioned above.